It has recently become usual practice to heat a fluid in a pipeline and thereby lower its viscosity, or elevate its pressure in order to raise the efficiency of its transport. As a result, there has grown a demand for a coated steel pipe which can withstand use in an environment having an elevated temperature of, say, 100.degree. C. and an elevated pressure.
A polyethylene-coated steel pipe which has hitherto been used in a pipeline for conveying petroleum, gas or water can only withstand use at a temperature not exceeding, say, 60.degree. C., since polyethylene softens at a higher temperature. Even a pipe coated with high-density polyethylene can only withstand use at a temperature not exceeding, say, 70.degree. C. or 80.degree. C., and cannot withstand continued use at 100.degree. C.
It is, therefore, necessary to explore a thermoplastic resin which can be used for coating a steel pipe to make it suitable for use at a temperature as high as, say, 100.degree. C. Polypropylene is most preferable as the least expensive thermoplastic resin that can be used for making a coated pipe by using any apparatus existing for making a polyethylene-coated steel pipe. The problems as stated at (1) and (2) below, however, have to be overcome to enable the practical use of polypropylene for coating steel pipes:
(1) The prolonged exposure of polypropylene to a high temperature in the presence of oxygen causes its degradation by thermal oxidation resulting in a cracked layer of polypropylene having only a seriously low level of corrosion resistance; and PA1 (2) If polypropylene is left exposed to an outdoor environment for a long time, the ultraviolet radiation which the sunlight contains causes its degradation by photo-oxidation resulting in a cracked layer of polypropylene having only a seriously low level of corrosion resistance.
Japanese Patent Application Laid-Open No. Hei 8-25560 proposes a steel material coated with a polypropylene resin as a solution to the problems as stated. The proposed solution, however, relies upon the addition as a principal material of a phenolic antioxidant having an ester group, or cyclic amide in its molecule, and it has been found that the proposed material has a sharp lowering in its resistance to deterioration by thermal oxidation in a humid environment having a temperature of 80.degree. C. or above, since the phenolic antioxidant is lost by hydrolysis in a humid atmosphere having a high temperature of 80.degree. C. to 100.degree. C.
Although a polypropylene-coated steel pipe, which is primarily intended for use under the ground, is not required to have a long period of weatherability lasting for as long as, say, 40 years, it is essentially required to have a short term of weatherability lasting for, say, five years, since it is likely to remain exposed to the light of the sun until the work of its installation is completed.
There is known a case in which a steel pipe coated with polyethylene containing about 2% by weight of carbon black has maintained weatherability for about 40 years. The addition of carbon black to polypropylene, however, brings about a serious lowering of its resistance to deterioration by thermal oxidation in a humid environment having a high temperature, whatever oxidation inhibitor may be added to it. Accordingly, there has been wanted a technique other than the addition of carbon black which is applicable to a polypropylene-coated steel pipe.
It is, therefore, an object of this invention to provide a polyolefin-coated steel pipe having both resistance to deterioration by thermal oxidation and weatherability, and also an effective process for manufacturing the same.